Copolypeptides with propargyl side chains were functionalized with different thiols (1-thio-β-D-glucose tetraacetate, 7-mercapto-4-methylcoumarin, and methyl-3-mercaptopropionate) by initiator/catalyst-free thiol–yne photochemistry. No more than 1.5 instead of ideally 2 equivalents of thiol could be added to the propargyl units even when the thiol was used in excess, attributable to steric constraints (close proximity of the reaction site to the copolypeptide backbone) or conjugation (extended aromatic system). Nevertheless, this procedure enables a facile and modular synthesis of highly functionalized homo- and hetero-functional copolypeptides.